Add a useDebouncedSave hook and delete Settings.stream

Two items from Stage 2 of `plan/17-refactor.md`.

`frontend/src/hooks/useDebouncedSave.js` replaces three copies of the same
debounce. `PlotPanel` and `ScenarioEditor` held identical per-key timer maps.
`ScriptEditor` held a single shared timer, so editing two fields inside the
same 600 ms window canceled the first save. The hook gives every key its own
timer, which fixes that.

`Settings.stream` was dead state. Nothing read it and every turn streams. This
removes the column, both schema fields, and adds migration 65 to drop it. It is
item S1 in `docs/self-review.md`.

Migration 65 needs a new guard. `_column_already_gone` is the counterpart to
`_column_already_there`: `create_all` builds the current schema, which is
already missing every dropped column, so a fixture that stamps an old version
and replays would fail on a column that is not there.

Verified: 549 backend tests pass, lint and build are clean, and migration 65
runs both ways, once against a database that still has the column and once
against one that does not.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014Dix4oGV3njgWRdu7P9t6r

frontend/src/hooks/useDebouncedSave.js

@@ -0,0 +1,17 @@
+// Delays a save until its field has been quiet for a moment.
+//
+// Every `key` gets its own timer. One shared timer cancels the pending save of
+// whatever was edited before it, so editing two fields inside the same window
+// saves only the second one.
+
+import { useRef } from 'react'
+
+function useDebouncedSave(delay = 600) {
+  const timers = useRef(new Map())
+  return (key, fn) => {
+    clearTimeout(timers.current.get(key))
+    timers.current.set(key, setTimeout(fn, delay))
+  }
+}
+
+export { useDebouncedSave }